Where is the Postgresql config file: 'postgresql.conf' on Windows?

You can find it by following this path

C:\Program Files\PostgreSQL\13\data

On my machine:

C:\Program Files\PostgreSQL\8.4\data\postgresql.conf

postgresql.conf is located in PostgreSQL's data directory. The data directory is configured during the setup and the setting is saved as PGDATA entry in c:\Program Files\PostgreSQL\<version>\pg_env.bat, for example

@ECHO OFF
REM The script sets environment variables helpful for PostgreSQL

@SET PATH="C:\Program Files\PostgreSQL\<version>\bin";%PATH%
@SET PGDATA=D:\PostgreSQL\<version>\data
@SET PGDATABASE=postgres
@SET PGUSER=postgres
@SET PGPORT=5432
@SET PGLOCALEDIR=C:\Program Files\PostgreSQL\<version>\share\locale

Alternatively you can query your database with SHOW config_file; if you are a superuser.